Liquid membranes are formed by placing a liquid in contact with a substantially immiscible fluid. Industrial use of liquid membranes has been limited by the costs and difficulties associated with the creation and destruction of stable liquid membranes. Currently, liquid membrane separation techniques provide an attractive alternative to traditional separation processes only when the target species must be extracted to very low concentration levels. For example, even low levels of metal ions or organic compounds present in industrial wastewater can often be reduced to lower concentrations through application of liquid membrane technology.
One method of avoiding problems associated with creating and maintaining stable liquid membranes involves the use of porous supports having pores into which a liquid can be introduced. Because of surface tension arising from the interaction of the interior surfaces of the pores and the introduced liquid, these so-called supported liquid membranes are much more stable than liquid membranes not formed around a solid support. Although supported liquid membranes avoid problems associated with stabilization of the liquid membrane, they suffer from other practical problems such as limited surface area and low rates of mass transport. In effect, the stability problems are minimized at the cost of greatly reducing the rate of mass transport across the supported liquid membrane. This reduction in mass transport rate particularly diminishes opportunities for utilizing supported liquid membrane technology to separate mixtures containing large biomolecules. Biomolecules such as enzymes, antibodies, and other proteins naturally have low molecular diffusion rates, making mass transfer of such biomolecules by diffusion through a supported liquid membrane too slow for the process to be commercially practicable.
One method of increasing the mass transfer rate of supported liquid membranes is a subject of U.S. Pat. No. 4,994,189 to Leighton et al., filed Sep. 30, 1988, which describes a method and apparatus for increasing transfer of a selected solute species across a supported liquid membrane. A porous support is permeated with a liquid that is oscillated at a frequency selected to enhance the rate of mass transport of the solute species across the liquid membrane. By appropriate selection of oscillation frequency, large molecules having normally low diffusivities can be transported across the supported liquid membrane at a greater rate than small molecules having normally high diffusivities. This seemingly paradoxical reversal in mass transport rates of a solute species through pores containing a liquid subject to oscillatory motion is a consequence of the interaction between the radial diffusion of the solute species in the pores and the dispersion of the solute species across the pores as a result of laminar flow of the liquid. To appreciate this interaction, consider molecules of the selected solute species entering the liquid contained in a pore at the center of the interface between a fluid containing the molecules and the liquid in the pore. The molecule is transported some distance along the length of the pore by the laminar flow of the liquid in the pore during the first half of the oscillation cycle, and absent any other effects would ordinarily be transported back to its entry point during the second half of the liquid oscillation cycle as the liquid flow is reversed. The rate of mass transport across the length of the pores would be unchanged by this oscillating liquid flow, remaining dependent only on the rate of diffusion through the length of the pore, since the effects of non-diffusive flow transport are cancelled out by the oscillatory nature of the liquid flow. However, the mass transport rate across the length of the pores can be increased for molecules having a particular rate of diffusion in the liquid, if the oscillation frequency of the liquid is selected so that on the average the radial diffusion of the molecules from the center of the pore to the pore walls can take as a minimum about as long as half of one oscillation cycle. In that case the molecules, on the average, could radially diffuse from the center of the pore to near the walls of the pore during the course of the first half of one oscillation. When the flow reverses during the second half of the oscillation cycle, the molecules will not move backward to their entry point as quickly, since the velocity of laminar flow of the liquid near the pore walls is much less than the velocity of laminar flow near the center of the pores. The molecules tend to remain in the volume of the pore at the side opposite its entry into the pore liquid, giving the molecule a better chance of diffusing out of the pore on that side. Of course, other molecular paths are also likely, but the combined effect of oscillating fluid motion together with radial diffusion in the pore spaces is to greatly increase the dispersion, and hence the mass transport, along the length of the pores.
At low oscillation frequencies, this enhancement in mass transport works best for solute species having low radial diffusion rates. Solute species having high radial diffusion rates can move back and forth between the center of the pore and the pore walls many times during the course of one oscillation, making the average mass transport rate essentially no different than that provided by simple diffusion through the pores. Because the magnitude of the enhancement in mass transport rate of the selected solute species in the oscillated system is so much greater than of a non-oscillated system or of other molecular species not having the same rate of radial diffusivity, large, normally slowly diffusing solute species can be transported across the membrane fast enough to make industrial separations feasible using supported liquid membranes.
However, one difficulty associated with the use of homogeneous porous supports having essentially uniform pore diameters through the support could reduce the ability to use such supported liquid membranes in industrial separations. If the oscillations are induced by cyclic pressure changes, there is a possibility that the pressure change could exceed the capillary pressure in the pores of the supported liquid membrane. This could result in the expulsion of the fluid from the pores, destroying the liquid membrane.
It is therefore an object of this invention to provide an improved porous support for a liquid membrane capable of maintaining membrane stability under conditions of varying pressure that would destabilize liquid membranes supported by a homogeneous porous support.
Another object of this invention is to provide a porous support structure into which fluids can be introduced to form a liquid membrane capable of promoting separation of chemical species in a mixture by selective enhancement of transport of a desired chemical species across the supported liquid membrane.
Yet another object of this invention is to provide an oscillated, supported liquid membrane wherein oscillation of liquid within the support is controlled by cyclic pressure variations of a fluid contacting the liquid in the support.
In accordance with the foregoing objectives, the present invention provides a heterogeneous support for a liquid membrane that includes first and third layers. Both the first and third layers have a plurality of outer pores respectively extending therethrough. Sandwiched between the first and third layers is a second layer that is also formed to have a plurality of pores extending therethrough. The second layer is positioned between the first and second layers to allow fluid communication between the pores of the first layer and the pores of the second layer, and to also allow fluid communication between the pores of the third layer and the pores of the second layer. The pores of the first and third layers have an average pore diameter that is selected to be greater than that of the pores of the second layer.
In preferred embodiments the average pore diameter of the pores of the second layer are selected to range from about one-half to about one-twentieth the average pore diameter of the pores of the first and third layers. Typically, the average pore diameter of the pores of the first and third layers are selected to range from about 5 micrometers to about 50 micrometers, and the average pore diameter of the pores extending through the second layer are selected to range from about 0.5 micrometers to about 5 micrometers. To increase the capillary pressure in the pores, the walls of the pores can be coated with or formed from a hydrophobic material if a hydrophobic liquid is to be introduced into the pores, or can alternatively be treated with or formed from a hydrophilic material if a hydrophilic liquid is to be introduced into the pores.
The heterogeneous support can form part of an apparatus for enhancing mass transfer of a chemical species in a first fluid through a supported liquid membrane by selective enhancement of the mass transfer rate of selected chemical species across the length of pores extending through the heterogeneous support. A first chamber containing the first fluid is separated from a second chamber containing a second fluid by the heterogeneous support. A heterogeneous supported liquid membrane is formed by at least partially filling the pore spaces of the heterogeneous support with a liquid that is selected to be substantially immiscible with the first fluid contained in the first chamber. The liquid can be introduced into the pores of the heterogeneous support by immersion of the heterogeneous support into a bath of the liquid or by other means for forcing the liquid into the pores. The liquid in the heterogeneous support is then caused to oscillate at a predetermined frequency. This oscillation can be driven, for example, by cyclic pressure changes in either the first or second fluid contacting the liquid that forms the supported liquid membrane. The liquid in the pores of the heterogeneous support reversibly flows in a laminar flow regime from the first layer to the third layer, passing through the pores of the second layer, as a result of these externally driven fluid oscillations. However, because the capillary pressure in the pores of the second layer is greater than the maximum pressure difference between the first and second fluids contacting the liquid in the pores of the second layer, those pores always remain filled with the liquid, in effect acting to "pin" the liquid membrane in the heterogeneous support. The frequency of this "sloshing" oscillatory motion of the liquid through the pores of the heterogeneous support is selected to enhance the mass transfer rate of chemical species from the first chamber through the pores of the first and third layers and into the second chamber.
One advantage of the present invention is that heterogeneous supports can readily replace conventional homogenous supports in laboratory or industrial processes.
Another advantage of the present invention is that liquid in a supported liquid membrane can be induced to reversibly flow through the support by cyclic pressure changes created in a fluid contacting the liquid without separating from the support. This property is useful because large scale and industrial separation processes generally have an existing capability for pressure modification, and imposition and control of sinusoidal or other oscillatory pressure variations is relatively easy.
Yet another advantage of the present invention is the ability to construct heterogeneous supports by preparing a laminate of commonly available porous layers having differing pores sizes. Commercially available porous supports having generally uniform pores suitable for accomodating and supporting a liquid membrane are widely available. Laminates consisting of an inner porous layer sandwiched between two outer porous layers having larger pore diameters than the average pore diameter of the inner layer can be readily constructed.
Other objects, features, and advantages of the present invention will become apparent with reference to the following written description of the drawings and the examples of particular embodiments of the invention.